This application is based on application Ser. No. 10-137664 filed in the Japan Patent Office on May 1, 1998, the content of which is incorporated hereinto by reference.
(a) Field of the Invention
This invention relates to a negative electrode in a lithium secondary battery, and more particularly to a carbonaceous material for a negative electrode in a lithium secondary battery which reduces an irreversible capacity of a first cycle and increases a discharge capacity of the battery.
(b) Description of the Related Art
In recent years, with the increasing performance and portability of electronic equipment, it is necessary for secondary batteries used in the equipment to have high levels of energy density. A lithium secondary battery comprising a lithium metal as a negative electrode, and a carrier compound for lithium ions as a positive electrode material having an oxidation potential differing from the negative electrode, is proposed as a high energy density. However, since such a battery has safety problems, now more commonly used is a lithium ion secondary battery having a negative electrode containing a carbonaceous material in which lithium ions undergo intercalation and deintercalation between carbon layers.
Since a lithium metal is not directly employed as the negative electrode in the lithium ion secondary battery using the carbonaceous material, the reaction between lithium of the negative electrode and electrolyte can be suppressed and a short circuit caused by the formation of dendrite on a surface of the lithium ions can be prevented. However, a capacity per gram is reduced according to the degree of intercalation the lithium ions undergo between the carbon layers. Accordingly, although it is necessary to use a carbon material able to intercalate/deintercalate a maximum amount of lithium ions, a theoretical capacity of only to 372 mAh/g is achieved in the case of the most highly developed graphite in the gaps between layers. In addition, irreversible capacity of about 10% occurs from a reaction between an organic electrolyte and a surface of the graphite or a reaction between the lithium ions and a cointercalated electrolytic material.
Accordingly, in order to increase the capacity of a carbon negative electrode, there is proposed a composite material enabling lithium intercalation at areas other than between carbon layers. The composite material is coated with metal alloyable with lithium. However, it is also problems that is hard to control the nature of the coating and to reduce the irreversible capacity.
The object of the present invention is to provide a carbonaceous material for a negative electrode of a lithium secondary battery which decreases an irreversible capacity and increases a discharge capacity of the battery. It is also an object of the present invention to provide a lithium secondary battery using the carbonaceous material.
These and other objects may be achieved by a carbonaceous material for a negative electrode of a lithium secondary battery, a surface of carbon particles of the carbonaceous negative electrode being coated with a porous nickel thin film, wherein a nickel adhesivity level for carbon is 0.05% by weight to 1.0% by weight.